A diffuser, in an automotive context, is a shaped section of the vehicle underbody which improves the vehicle's aerodynamic properties by enhancing transition between high-velocity airflow underneath the car and the significantly slower free-stream airflow of the ambient atmosphere. The diffuser generally affects pressure under the vehicle body as the vehicle moves through the air to increase effective downforce on the vehicle.
The diffuser is typically located at the aft part of a vehicle underbody and works by accelerating the velocity of the airflow underneath the vehicle. The diffuser works by providing a space for the underbody airflow to decelerate and expand into an area covered by the vehicle while air density remains constant. Specifically, the diffuser uses Bernoulli's principle, according to which fluid pressure decreases as velocity of the fluid increases. Therefore, the diffuser causes the pressure below the vehicle to be lower than on the side and above the vehicle, thereby generating a measure of downforce.
When a diffuser is used, the air flows into the underbody from the front and sides of the vehicle, following which the air accelerates and reduces pressure under the vehicle. A suction peak is generated at the transition from the underbody to the diffuser. The diffuser then eases the high velocity underbody air back to the velocity of the free-stream airflow moving past the vehicle and also helps fill in the wake area immediately behind the vehicle. As a result, the diffuser reduces drag and increases downforce on the vehicle by making the whole vehicle underbody a more efficient downforce producing device. Additionally, the diffuser imparts upward momentum to the air aft of the vehicle, which further increases downforce on the vehicle.